1. Field of the Invention
This invention relates to improvements in a device for focus detection or distance detection in which one or more images of a target formed by an optical system are received by photoelectric light receiving means to obtain a photoelectric output regarding the imaging condition of the image of the target and this photoelectric output is electrically processed through a processing circuit to thereby provide an electrical output representing the focus adjusted condition of the optical system with respect to the target or the distance to the target.
2. Description of the Prior Art
There have heretofore been proposed numerous devices which are adapted to photoelectrically detect the focus adjusted condition of an optical system with respect to an object or the distance to the object. As the principle of focus detection in such devices, there has often been adopted a method in which a photoelectric light receptor is disposed in a predetermined image formation plane of a picture-taking optical system or a detection optical system operatively associated with the picture-taking optical system so that the degree of clearness of the image of an object is detected by the photoelectric output of the photoelectric light receptor to thereby know the focus adjusted condition of the picture-taking optical system with respect to the object; or a method whereby the degree of deviation between two upper and lower split images formed by a split image optical system is detected by the output of a photoelectric light receptor to thereby know the focus adjusted condition of the picture-taking optical system with respect to an object; or a method in which the measuring optical system is designed as a so-called base line range finder type optical system and a part thereof is operatively associated with the picture-taking optical system, whereby two images of an object are formed with an amount of relative deviation corresponding to the deviation of the adjusted position of the picture-taking optical system with respect to the distance to the object and the amount of relative deviation between the two images is detected on the basis of the output of the photoelectric light receptor to thereby know the focus adjusted condition of the picture-taking optical system with respect to the object. On the other hand, as the principle of distance detection, there has often been adopted a method whereby two images of an object are formed by a fixedly disposed base line range finder type optical system with an amount of relative positional deviation corresponding to the distance to the object and the amount of relative deviation between the two images is detected on the basis of the photoelectric output from a photoelectric light receptor to thereby know the distance to the object; or a method in which a part of the aforementioned optical system is made movable to enable one image to be arbitrarily moved relative to the other image and the point whereat the amount of relative deviation between the two images is eliminated is detected by the output of the photoelectric light receptor to thereby know the distance to the object from the position of the movable part of the optical system at said point.
In any case, such devices are very often utilized as the automatic focus detection devices or further as the automatic focus adjusting devices, in optical instruments such as cameras or the like and in such case, the detecting operation thereof must be sufficiently ensured even for objects of very low brightness, with the area to be photographed by the camera or the like being taken into consideration.
On the other hand, however, the light sensing portion of the photoelectric light receptor is often formed of Si(silicon) or CdS(cadmium sulfide) or like material and accordingly, the detectable brightness level is naturally limited by the conditions determined by the physical properties of these materials and when the noise signals or the like generated by the subsequent processing electrical circuit are taken into account, the substantial limit of detection is usually further narrowed.
In view of such inconveniences, according to, for example, U.S. Pat. No. 3,813,679 (granted to Hasegawa et al. for an invention entitled "FOCUS DETECTOR DEVICE" assigned to Canon K.K.), U.S. Pat. No. 4,002,899 (granted to Stauffer for an invention entitled "FOCUS DETECTING APPARATUS") or Japanese Laid-Opened Patent Application No. 55658/1977 (entitled "DISTANCE DETECTING DEVICE"), there have been proposed some types of device in which an illumination source for additionally illuminating an object is provided so that when the brightness of the object is low, the object may be illuminated automatically or by manual operation to thereby enable the detection with respect to the object of low brightness. However, in these proposals, an object is uniformly illuminated by an illumination source and therefore, much light energy is projected to other than the view field to be actually detected and accordingly, the power consumption required for driving the illumination source is unnecessarily increased to cause a great loss of the power source, and in addition, however high brightness of illumination is effected for an object of uniform brightness, namely, an object of very little contrast, the brightness level of the entire object is only increased and only uniform light is received by the photoelectric light receptor and thus, for such an object, it is impossible in principle to achieve focus detection or distance detection even if an additional illumination is provided.